Device for monitoring biological signals from patients, while an electro-surgical appliance is being simultaneously used

ABSTRACT

The present invention relates to a system for use in monitoring a biological signal from a patient while an electro-surgical appliance which applies a high r.f. voltage, is being simultaneously used. The signals from a number of transducers are transmitted to an amplifier preferably an ECG amplifier with an input-filter. The amplifier is situated in a Faraday cage which is connected to one of the transducers. The filter is a screened HF-filter and the filter together with the Faraday cage is enclosed in an earth connected grounded container or screen. As a result radiation from the Faraday cage is substantially eliminated.

The invention relates to a device for use in monitoring a biologicalsignal from a patient while an electro-surgical appliance which appliesa high r.f. voltage is being simultaneously used.

An ECG device is known from U.S. Pat. No. 3,915,154. The screen orshield of the cable connected to the ECG electrodes is linked to ascreen surrounding the ECG amplifier. This device is suitable forsuppressing noise voltages emanating from the mains. However, ECGsignals are not detected in this device in the case where an r.f. signalhaving a magnitude of several hundred volts is fed to the tissue.

Even if an r.f. filter is interposed before the ECG amplifier, it willnevertheless be impossible to detect the ECG signals reliably enough. Infact radiation outside and inside the screen will affect the operationof the device.

According to the present invention, there is provided a device whichmonitors a biological function of a patient during use of anelectro-surgical appliance which delivers high r.f. voltages to thetissue under consideration, comprising at least one sensor which, whenlinked to the patient, provides a signal representative of thebiological function, which signal is supplied via a screened r.f. filterto an amplifier, the filter and amplifier being disposed within aFaraday cage which is connected to a reference potential and insulatedfrom a surrounding earthed or grounded container. Thus, in a deviceaccording to the invention, the radiation emanating from the filterinductors is screened, and so is radiation emanating from the Faradaycage.

It is advantageous for the inductors of the filter at the input of theamplifier to be separately screened.

Preferably the device is so arranged that the capacity between theFaraday cage and the external screening container, which is earthed whenthe device is in use, is in the range 30 to 60 pF. As a result the r.f.signal is to some extent short-circuited, but the inflowing r.f.currents are not so large that the r.f. voltage drop in the tissue ofthe patient becomes excessive.

Where the transducer means requires a voltage feed, as opposed to ECGelectrodes which require no applied voltage, this voltage feed may beapplied via a further filter or filters whose inductors are fitted andscreened in the screened Faraday cage, in which case the power supplyused may be the power supply which is used for energizing the amplifier.

A specific embodiment of a device according to the present inventionwill now be described by way of example with reference to theaccompanying drawing wherein:

FIG. 1 is a circuit diagram of circuitry, including an ECG amplifier,fitted in a screened Faraday cage.

One form of electro-surgical appliance, with which a device according tothe present invention can be used, comprises a radio-frequency source ofwhich one electrode having a relatively large surface area is arrangedto be coupled to the back of a patient, and of which the other electrodeconsists for example of the knife used by the surgeon. During anoperation, bleeding is stopped by sending r.f. currentsamplitude-modulated at 50 Hz through the tissue under the control of thesurgeon by use of a pedal-operated control device. In the first place,coagulation occurs, in the second place there is a certain amount ofcarbonisation, and in the third place the tissue dries out. All thesethings contribute to stopping the bleeding. The applied voltage ispreferably in the range 300-400. The frequency is preferably in therange 750 kHz-1.6 MHz; such a frequency does not upset the nerve-cellsof the tissue.

The device illustrated in this embodiment comprises electrodes ortransducer means which, for ECG monitoring, may be one referenceelectrode and two ECG electrodes. The reference electrode is connectedto a metal container 10 which acts as a Faraday cage. The two activeelectrodes are connected to the input terminals 2,4 of a symmetricallyconstructed filter 6, which provides an attenuation of about 70 dB at 1MHz. The filter has inductors L1 and L2 which are individually screenedas indicated in FIG. 1. The output of the filter is connected to an ECGamplifier 8 (the attenuation by 70 dB prevents the amplifier from beingoverloaded by the modulation signal of the r.f. signal), which is fittedin a part 11 of the Faraday cage 10, the whole of which is enclosed inand insulated from a screening container 17. The capacity between thecage 10 and the screening container 17 is in the range 30-60 pF. Thisarrangement results in the r.f. signal being to some extentshort-circuited, thus limiting the change in potential of the cage 10.The ECG amplifier 8 is of the fully symmetrical type, with a gain ofabout 40 times over a frequency range extending to below one Hz. Theamplifier comprises matched resistances, thus avoiding any resistanceadjustment. Following the ECG amplifier 8 is an attenuator including twodiodes parallel D1, D2 coupled in opposition, which has the effect,should overloading occur, of clamping the signal level. The outputsignal of the ECG amplifier 8 (of about 41 mV) is fed via a capacitor C6to a self-oscillating multivibrator 12.

When the device is energized, the multivibrator acts in the followingmanner:

Feedback to the positive terminal--see FIG. 1--sets up an initialvoltage of about 66 mV (10 V 150) at the positive input. The capacitorC7, which has zero volts across it at the moment when the device isswitched on, is charged up via the feedback resistor R5. After a certaintime, the capacitor C7 reaches 66 mV, and then the output changesabruptly from its positive level to its negative level. The potential ofthe positive input is then at -66 mV, with the result that the capacitorC7 then charges towards -10 V until the voltage of -66 mV is reached andthe cycle then repeats continuously. The frequency of oscillation isabout 1 kHz.

As mentioned, the output signal of about 41 mV of the ECG amplifier isfed to the input of the multivibrator 12. This is to modulate thecharging of the capacitor C7 and thereby modulate the pulse-width of themultivibrator 12. This modulation depends unambiguously on the ECGsignals. A coupling capacitor C8, having a value of 2.2 nF,differentiates the output signal of the multivibrator (rectangular-wavesignal) and passes only the flanks to a light-emitting diode D3. As aresult, only a brief glimmer of light occurs at each change of state ofthe multivibrator 12. This manner of modulation saves enegy.

A d.c. voltage supply arrangement 14 for the ECG amplifier comprises arectifier bridge fitted in the cage 10. To the rectifier bridge isconnected a toroidal winding which is connected via a single turn 15 toa toroidal winding of an inverter, shown in FIG. 3, disposed outside thecage 10. The capacity between the toroidal windings is about 0.7 pF andthe capacity of each toroidal winding is about 1.4 pF.

The light-emitting diode D3 transmits light to a phototransistor of thedemodulator via a light pipe LP, and the capacitance between diode D3and the phototransistor can be as small as desired by appropriatelyselecting the distance between them.

An additional advantage is that the transmission path of the powersupply is different from the transmission path of the output signal.This prevents any cross-modulation which might otherwise occur. It isalso an advantage that the capacity is at its lowest in the transmissionpath of the output signal.

The amplitude of the r.f. signal may be optionally reduced byappropriately positioning the ECG electrode. Since the patient is alwaysreceiving and transmitting a signal at 50 Hz, the positioning of theelectrodes may be checked with this signal. By measuring the strength ofthe 50 Hz signal received at the electrodes, it can be determinedwhether or not the skin under the electrodes must be further abraded(the d.c. or low frequency resistance must be below 5 kΩ), and whetherthey must be fitted such that they are on a line at right-angles to ther.f. current. When the electrodes are properly fitted, a filter whichfilters off the remainder of the 50 Hz signal is coupled into the outputstage (the demodulator).

Whereas the above described device has been described in connection withmonitoring of ECG signals, it will be appreciated that it can be readilymodified as appropriate for monitoring other biological signals, forexample those representing blood pressure and temperature, EEG and EMG.In the case of monitoring blood pressure and temperature signal, thevoltage on the secondary side of the transformer in the Faraday cage isreduced, and the voltage is fed out to the transducers via a filter(which is likewise fitted in the cage and has individually screenedinductors).

I claim:
 1. A device for monitoring a biological function of a patientduring use of an electro-surgical appliance which delivers high r.f.voltages to the tissue under consideration, the device comprising areference electrode and two active electrodes adapted to contact thebody of the patient, thereby providing a signal representative of thebiological function, a signal amplifier having two input terminals, ashielded conductor and a filter for connecting the electrodes with theamplifier, the reference electrode, and the conductor shield beinginterconnected, and each of the active electrodes, being connected to anamplifier input terminal, an ungrounded shield surrounding said filterand said amplifier, and a grounded container surrounding and insulatedfrom the ungrounded shield.
 2. A device as claimed in claim 1, whereinthe filter incorporates an inductor and in which the inductor isprovided with an individual shield.
 3. A device as claimed in claim 1,characterized in that the r.f. voltage has a frequency in the range from750 kHz to 1.6 MHz and in which the capacitance between the ungroundedshield and the grounded container is in the range of 30 to 60 pF.